A fundamental study on the ductility of high strength steels under impact deformation is carried out to investigate the effect of the local ductility of various materials on crash performance. In this study, newly developed 980 and 1180 MPa grade steels are investigated to clarify their advantages in term of crash performance compared to conventional DP (Dual Phase) steels. The features of the developed steel, named as jetQ are higher yield strength and higher local ductility due to an optimized microstructure by the quenching and partitioning process (QP) [1, 2]. The bending test according to VDA 238-100 is performed while observing the fracture propagation during the bending test. Fracture strain in the tensile tests is evaluated by a three-dimensional shape measurement technique for the fracture surface.
Both three-point bending tests and axial impact tests are performed to evaluate the crashworthiness of different types of steel. The three-point bending test simulates the “structural bending and intrusive deformation” that occur in the automotive side impact test. In the axial impact test, the “folding deformation” and energy absorption during the automotive front crash test are simulated in a laboratory testing machine. In this study, the fracture behavior and energy absorption performance of the high strength steels are investigated under both deformation modes.
Based on the experimental results, the effect of the local ductility of the high strength steel on the risk of material fracture is discussed. The experimental results show that high strength steels with higher ductility of thickness strain can improve crash deformation with lower risk of crash fracture. The jetQ steels of grades over 980 MPa have excellent local ductility compared to conventional DP sheets. The jetQ steel also shows higher crash performance due to its higher yield strength. Considering these advantages, the applicability of the jetQ steel to automobile body structures is discussed from the viewpoint of light weight body structures.
